In an MRI apparatus, an electromagnetic-wave high-frequency signal is irradiated onto a subject arranged in a homogeneous magnetostatic field generated by a magnetostatic field magnet to excite a nuclear spin in the subject and to receive an electromagnetic-wave nuclear magnetic resonance signal generated from the nuclear spin, and signal processing is performed to image the subject. Irradiation of the high-frequency signal and reception of the nuclear magnetic resonance signal are performed by a device, called an RF antenna or an RF coil, which transmits or receives an electromagnetic wave having a radio frequency (RF).
As the type of RF coil, there are a transmission antenna which performs only transmission, a reception antenna which performs only reception, and a transmission and reception antenna which performs both transmission and reception. In an MRI apparatus for human imaging which has magnetostatic field strength equal to or less than 3 tesla, in many cases, a large transmission antenna having a cylindrical shape or a disk shape and a comparatively small reception antenna having various shapes, such as a sheet-like shape and a cylindrical shape, are used in combination.
As an example of a cylindrical transmission antenna, there are a transmission antenna called a bird cage type (for example, see NPL 1 and PTL 1) and a transmission antenna called a TEM type (for example, see PTL 2 and PTL 3). In these transmission antennas, usually, 16 to 32 rod-like conductors arranged in parallel with a cylindrical center axis called a rung (a crosspiece or a crossbar of a ladder) are provided along the lateral surface of the cylinder. The cylindrical transmission antenna is used in an MRI apparatus called a tunnel type. In the tunnel type MRI apparatus, a cylindrical magnetostatic field magnet is arranged to form a tunnel, a subject enters the tunnel in a state of being laid on a bed, and imaging is performed.
As an example of a reception antenna, for example, there is an example where a conductor is bent in a loop shape (for example, see PTL 4), an example where a conductor is bent in a shape of a figure eight (for example, see NPL 2), or the like. Since the reception antenna is arranged closer to the subject than the transmission antenna, while sensitivity is high compared to a volume antenna, in many cases, a sensitive region is partially narrowed.